1. Field of the Invention
This invention relates to an analytical method and device for atmospheric analysis and, more specifically, to a method and device for detecting low concentrations of organophosphorus material in the atmosphere.
2. Description of the Prior Art
Environmental chemistry has been concerned with detection and measurement of organic material in the atmosphere or aqueous solutions. Techniques such as gas chromatography, infrared spectroscopy and chemically sensitive electronic devices have been applied (Process Instruments and Controls Handbook- Considine, P. 6-1-6-213 ). Water acts as an interferant in each of these analytical methods; therefore, it is necessary to remove the water before analysis can be done (Identification & Analysis of Organic Pollutants in Water-Keith, p.113; Modern Practice of Gas Chromatography -- Grob, p.135). It is advantageous to use an analytical method for organic material which does not require removal of water.
The Raman spectrometer detector has been used to analyze organic material in the presence of water. The Raman spectra results from scattering of source light directed on the sample at an angle. Water is transparent to laser light used in Raman spectroscopy since it exhibits very weak Raman scattering and, therefore, is a good solvent for material to be analyzed by Raman spectroscopy. In addition, since the Raman effect is exhibited most strongly by nonpolar homoatomic groups, such as carbon-carbon bonds, aqueous solutions of organic compounds are good candidates for Raman spectroscopic analysis as shown in FIG. 1.
Prior art describes uses of Raman spectroscopy for atmospheric and chemical analysis but does not include condensation as a method to concentrate low quantity components for analysis. Schnell (U.S. Pat. No. 4,620,284) teaches comparison of the digitized spectra of an unknown to reference spectra by means of a computer. Leonard (U.S. Pat. No. 3,723,007) teaches remote quantitative atmospheric analysis by subjecting the atmosphere to pulse laser radiation to determine distance and composition from the Raman scatter. Falconer (U.S. Pat. No. 4,071,298) teaches detection of the presence and the nominal size of aerosol particles by Rayleigh (unshifted) scattered light and their composition and mass by Raman and fluorescent (shifted) scattered light. Dudenbostel (U.S. Pat. Nos. 2,527,121; 2,527,122) teaches determination of the percentage of aromatics ("121") and olefins ("122") in a hydrocarbon mixture by determining the presence and strength of a spectra within a certain wavelength. However, sample preparation has not been a significant factor since detection of low concentrations was not an objective.
One drawback in the use of Raman spectroscopy is the weak spectra generated. Use of a laser can strengthen the spectra but for a sample in which the concentration of the material for which the analysis is being done is small, the problem still exists. Thompson (U.S. Pat. No. 3,906,241) addressed this problem by designing a sample container which, by means of fiberoptics, passes the light beam through the sample several times to strengthen the spectra. Leonard (U.S. Pat. No. 3,723,007) used photomultipliers to strengthen the signal.
Cold trap technology has been applied to gas concentration. Slobod (U.S. Pat. No. 2,512,040) teaches gas separation by cooling a gas mixture with liquid nitrogen to recover gaseous hydrocarbons by freezing them out of the noncondensable gases. Zaikowsky (U.S. Pat. No. 2,370,703) teaches preparation of samples for analysis by separating a gas mixture into gas and liquid components by collecting the gas and condensate in successive stages of freezing and revaporizing. However, the prior art does not suggest a method and device for screening the atmosphere for low concentrations of organic compounds in the range of 1 to 500 parts per billion.
The detection of low concentrations of organophosphorus compounds has application in the analysis of industrial and agricultural wastes and chemical warfare agents. Even at low concentrations, these compounds can be toxic. It has been demonstrated that detection of an organophosphorus compound, such as dimethylmethylphosphonate (DMMP), is possible with Raman spectroscopy ("Raman Spectroscopy of Chemical Agents and Simulants"--Christesen, Heyl, Proceedings of the 1985 Scientific Conference on Chemical Defense Research, p.707.) The problem has been to detect concentration as low as 1-500 parts per billion.